Process for increased olefin recovery in a petroleum refinery



prxl l0, 1951 D. M. LITTLE l PROCESS EOR INOREASED OLEEIN RECOVERY 1N APETROLEUM REEINERY Filed oct. 25, 1948 Patented Apr. 10, 1951 PROCESSFOR INCREASED OLEFIN` RECOV- ERY IN A PETRGLEUM'REFINERY i Donald M.Little, Bartlesville, one., assigner to Phillips Delaware PetroleumCompany, a corporation of Application October 25, 1948, Serial No.56,453

6 Claims. l

This invention relates to thetreatment of hydrocarbons. In one of itsmore specirlc aspects it relates to the separation of C5 and lighter by"drocarbons from a cracking furnace eiuent. In still another of its morespecific aspects it relates to the recovery of C3, C4 and C5 oleiinsfrom a mixture of hydrocarbons containing the same. In `still another ofits more specic aspects it relates to van improved process for therecovery of Ca, C4. and C5 olens from a cracking furnace eiiluent. Inyet another of its more specific aspects it relates to an improvedmethod of operating an integrated refining process comprising oleiinfeed preparation, alkylation and recovery of products.

The processes of reforming, polyforming and thermal cracking of lightoil stocks for the production of gasoline are widely used in thepetroleum industry. `In these processes an appreciable quantity ofolenic material is formed. Cx, Ci and C5 olens are used in thewell-known processes of alkylation, isomerization, chemical synthesis,etc. Many naphtha reforming and/or polyforming units were built or putinto operation before it became advantageous to recover olefins formedduring the reforming and/or polyforming processes. In the usualreforming or polyforming unit the furnace eilluent is separated into afuel oil fraction, a gas oil fraction, a light oil side draw fraction, agasoline fraction and a gaseous fraction containing substantially C5 andlighter hydrocarbons. The gasoline fraction is stabilized to provide agasoline fraction suitable for motor fuel or for blending purposes. Manyexisting units were designed merely to perform the operation ofstabilizing gasoline with no thought of recovering Ca, C4 and C5 olensto feed an alkylation unit, with the result that many Ci and C5 olensare contained in the stabilized gasoline fraction. The usual existingstabilizer will not make a good separation between the olefin-containingfraction and the C6 and heavier hydrocarbons which predominate ingasoline because the stabilizer will not handle the liquid loadnecessary for sufficient fractionation. The stabilizer-Will not handlethe reflux necessary to recover lall the available C4 and C5 olens. As aresult, C4 and C5 olens are lost for alkylation, isomerization, chemicalsynthesis, etc. The usual separation and recovery system, used inconjunction with a naphtha fraction reforming or polyforming operationor with -a light oil cracking operation, contains a .gasolineaccumulation zone, usually referred to as a gasoline accumulator, Thegasoline and most of the lighter hydrocarbons formed in the cracking foperation are fed into this gasoline accumulation zone. In mostoperations a gas comprised predominantly of C5 and lighter hydrocarbonslis taken overhead from the gasoline accumulator; however, theaccumulator acts only as a flash zone and no sharp separ-ation is made.The gasoline stabilizer is fed with the liquid in the gasolineaccumulator which contains appreciable quantities of C3, Ci and C5oleflns. The gasoline stabilizer is Operated so as to take overhead lasmany of the C3, C4 and C5 olens as possible. I have found that byintroducing a normally gaseous hydrocarbon fraction into a gasolineaccumulator, or into the feed to an accumulator, one can recover more ofthe Ca, C4 and C5 olefins in the overhead from the gasoline stabilizerand the gasoline accumulator, thereby making more olens available forrecovery in a light hydrocarbon n recovery system,

It is an object of this invention to provide an improved method ofseparating a mixture of hydrocarbons.

`Another object of this invention is to provide an improved method ofrecovering C3, Ci and C5 oleflns from the effluent loi a naphthareformer or polyformer or from a light oil cracking furnace.

It is another object of this invention to provide an improved method ofoperating a gasoline recovery system so as to recover more Cs, C4 andVC5 olefins. Y Y y Still another object of this invention is to provide,anY improved process of operating an integrated refining operationcomprising olefin fecd preparation, alkylation and recovery ofunreactecl hydrocarbons so as to increase the amcunt of Cs, Ci and C5oleflns available for alkylation.

Other objects of my invention will become apparent, to one skilled inthe art, from this disclosure.

My invention is particularly applicable to a process wherein a naphthafraction, such as a straight run naphtha fraction., is reformed orpolyforlned. Also, it is applicable to processes Where other light oilstocks are cracked so as to `form gasoline and lighter hydrocarbons.rlhe drawing is a schematic flow sheet diagrammatically showing apreferred embodilllent of my invention. The following discussion of vmyinvention applies to reforming-or polyforrning a straight run naphthafraction, as

a preferred example of my invention. A straight run naphtha fraction isfed to a reformer or polyformerv represented by heating zone 2 throughline 3. A ley-pass line 4 is provided so that part of the feed may bepreheated in heat exchanger G by interchange with the effluent fromheating zone 2. As will be appreciated, this heat interchange is purelyoptional and only acts to cool the furnace eluent to the temperaturedesired for the first fractionation step. The furnace effluent is fed toa depropanizer represented by fractionation zone I which operates tocarry overhead a portion of the light hydrocarbons produced in thereforming or polyforming furnace. In carrying` out my inventionfractionation zone I8 is operated at a top temperature of from 190C F.to 230 F., and at a pressure of from 300 to 360 pounds per square inchgauge. The vapors from the top of fractionation zone I0 are cooledand/or condensed in condenser I4 and passed to accumulator I8.predominantly of H2 and CH4 is vented from accumulator I8. Part of theliquid hydrocarbon fraction in accumulator I8 is passed back tofractionation zone I0 as reflux and the remainder, which is ahydrocarbon fraction comprised predominantly of C2, Cs and C4hydrocarbons, is passed through line 8 to a light hydrocarbon recoverysystem tied into the system or to an extraneous light hydrocarbonstream. A hydrocarbon fraction comprised predominantly of C4 and heavierhydrocarbons is Withdrawn from the bottom portion of fractionation zoneI0 as bottoms product and passed through line 9 to a ash zone 26. Thisbottoms product is heated in heating zone 22 before entering flash zone2'6. The fuel oil formed in heating zone 2 and present in the effluentfrom heating zone 2 is withdrawn from flash zone 26 through line I5. Thehydrocarbons lighter than the fuel oil fraction are taken overhead fromflash zone 26 and are passed to fractionation zone 30 through line I3.Fractionation zone 30 is employed to separate gas oil from the gasolineformed in the cracking operation. A clay tower to polymerize dioleflnsand gum forming constituents and a polymer separator (tower andseparator now shown) can be used in conjunction with fractionation zone30 to separate out polymers formed in the cracking operation and in theclay tower. This, of course, is optional at this stage of the process.Fractionation zone 30 is operated with a top temperature of from 375 to450 F., and at a pressure of from 50 to 10 pounds per square inch gauge.tionation zone 30 are condensed and cooled in condendenser 34. Part ofthe liquifled gases from the top of fractionation zone 30 are used asreflux and the remainder are'passed through line I2 into a gasolineaccumulation zone represented by accumulator 38. If a clay tower andpolymer separator are used to treat the overhead vapors fromfractionation zone 30, fractionation zone 30 can be refluxed with theliquid hydrocarbons in gasoline accumulator 38, that is, accumulator 38may serve as a reflux accumulator for fractionation zone 30. Thehydrocarbon fraction taken overhead from fractionation zone 30 andpassed into gasoline accumulator 38 contains the gasoline and lighthydrocarbons not taken overhead in fractionation zone IG which wereformed in the cracking operation. Substantially all of the C4 and C5olens which were formed in the cracking operation are contained inl thefraction passed to accumulator 38. A light hydrocarbon fraction, such asa hydrocarbon fraction comprised predominantly of C3 hydrocarbons, isadded either to the feed line I2 to accumulator 38 or directly toaccumulator 38. Either method of adding the light hy- A gaseous fractioncomprised The gases leaving the top of fracdrocarbon fraction will givethe desired results. A separation of vapor and liquid present inaccumulator 38 is made. An overhead fraction comprised predominantly ofC5 and lighter hydrocarbons is passed through line 2'I to a lighthydrocarbon recovery system. A liquid hydrocarbon fraction is withdrawnfrom the bottom portion of accumulator 38 and passed through line 25 toa gasoline stabilization zone or stabilizer represented by fractionationzone 42. Gasoline accumulator 38, is operated at a pressure of from 8 to16 pounds per square inch gauge and at a temperature from F. to 120 F.and operates primarily as a liquid-vapor separator, or flash zone, withno attempt at fractionating. The liquid hydrocarbon fraction Withdrawnas bottoms from accumulator 38 contains substantially all of thegasoline formed in the cracking operation and some Cs,-C4 and C5 olefinsalong with other C5 and lighter hydrocarbons. Stabilized gasoline iswithdrawn from the bottom portion of fractionation zone 42 through line29. Hydrocarbon vapors containing a substantial portion cf the C5, andlighter hydrocarbons contained in the feed to fractionation zone 42 arewithdrawn overhead from fractionation zone 42 and condensed in condenser46 which discharges to reflux accumulator 50. Part of the condensedhydrocarbons are used to reflux fractionation zone 42 and the remainderare passed through line 3i to a light hydrocarbon recovery system. Thegasoline stabilizer represented by fractionation zone 42I is operatedWith a top ternperature of from 140 F. to 180 F. and at a pressure offrom 5 to 13 pounds per square inch absolute. The withdrawn overheadhydrocarbon fractions from fractionation zone IIJ, separation zone orgasoline accumulator 3B and fractionation zone or gasoline stabilizer 42are passed to a light hydrocarbon recovery zone which is represented onthe diagrammatic flow sheet, by fractionation zones 54, 60 and 66. Thelight hydrocarbon recovery system just referred to is used to recoverC3, C4 and C5 oleflns to use as reactants in an alkylation processrepresented by alkylation reactors 12, acid settling zone 16 and acidstripping zone 80. In fractionation zone 54 a cut is made between C5hydrocarbons and gasoline present in the feed streams to the lighthydrocarbon recovery system. C5 and heavier hydrocarbons carried over inthe overhead from fractionation zone 42 and accumulator 38 are removedas bottoms product from fractionation zone 54 via line 39. The overheadfrom fractionation zone 54 containing C5 and lighter hydrocarbons ispassed through line 31 to fractionation zone 60. The overhead fromfractionation zone 54 may be deethanized if desired (means to do thisnot shown on the flow diagram). C3. C4 and C5 oleflns are taken overheadfrom fractionation zones 60 and 66 Via lines 58 and 62 respectively, andare passed through line 55 to alkylation reactors 12 which are chargedwith isoparafiins through line 51, with new acid through lines 59 and69, and with recycle acid through lines and 69 and/or 6I and 69. Ifdesired, C3 and C4 olens may be withdrawn from the system through lines43 and 5I and/or 43, 56 and 53. C5 olefins may be Withdrawn from thesystem via lines 45 and 49 and/or 45, 52 and 53, if desired. A. C5bottoms fraction from fractionation zone 60 is passed via line 4I tofractionation zone 66. Pentane bottoms from fractionation zone 66 isremoved via line 41. Acid settling zone 16 fed through line 63 and acidstripping zone 80 fed through line 61 are used to separate alkylate andunreacted parailins from the reaction mixture.

A mixture of alkylate and unconverted reactants is withdrawn from acidstripping zone 8D through line TI and passed to fractionation zone 84.The alkylate formed in the alkylation reactors is withdrawn from thebottom portion of fractionation zone 82B through line l1. Theunconverted parailns are recovered as represented by side cuts takenoli` through lines 'I3 and 15. A light hydrocarbon fraction comprisedpref dominantly of C3 hydrocarbons is taken overhead from fractionationzone 85. This light hydrocarbon fraction may be recycled to the gasolineaccumulation zone 38 through line I9 entering line I1. t may also bepassed directly to the overhead from fractionation zgne 30 by passing itto line I2. If desired, the overhead from fractionation zone 84 may bewithdrawn from the system through line 8l. It is to be understood thatthe foregoing discussion of my invention is given only as a preferredmethod of operation and the scope of my invention is not to be undulylimited by said foregoing discussion. The drawing is only a diagrammaticow sheet used in order to enable me to clearly point out my invention.One skilled in the art will appreciate that various modifications andarrangements of valves, pumps, auxiliary equipment, etc. can be made forany particular plant carrying out my invention.

Example The temperatures, pressures, quantities, equipment, etc.referred to in the following example are only illustrative of myinvention and are not to be deemed to unduly narrow the scope of inletpressure was 2-200 pounds per square inch gauge and the inlettemperature of the straightrun naphtha was 746 F., the straight-runnaphtha having been preheated by the furnace efliuent in a heatexchanger represented by heat exchanger 6. The temperature of thefurnace eiiluent was 970 F. The furnace effluent was.

cooled to 490 F. and passed into a depropanizer represented byfractionation zone I0. Said depropanizer was operated at a top pressureof 330 pounds per square inch gauge and a top temperature of 210 F. Thepressure drop over the depropanizer was 5 pounds per square inch. Agaseous fraction comprised predominantly of H2 and CHl was vented fromaccumulator I8 at a rate of 2081 M. C. FJD. The depropanizer wasrefluxed with a liquid hydrocarbon stream from accumulator I8 at a rateof 5,325 B./D. A hydrocarbon fraction comprised predominantly of Cz, C3and C4 hydrocarbons was withdrawn from accumulator I8 through line 8 asoverhead product. A `hydrocarbon fraction comprised predominantly of C4and heavier hydrocarbons was withdrawn from the bottom of thedepropanizer at a rate of 10,000 B./D. and passed through line 9 to aflash zone 26. lI'his bottoms product was heated from 546 F. to 692 F.in a feed heater represented by heating zone 22. Flash zone 26 wasoperated at a pressure of 65 pounds per square inch gauge and at atemperature of 670 F. A fuel oil product was withdrawn from the CTIbottom of flash zone 26 at a rate of 228 B./D. and passed through lineI5 to storage. The overhead gas from flash zone 26 was passed tofractionation zone 30 through line I3. Fractionation zone 30 wasoperated at a top temperature of 423 F. and at a top pressure of 61pounds per square inch gauge. The pressure drop across the fractionatorrepresented by fractionation zone 30 was approximately 5 pounds persquare inch. A side stream consisting of hydrocarbons boilingv in thekerosene range was withdrawn from fractionation zone 30 at the rate of653 B./D. Gasoil was Withdrawn from the bottom of fractionation zone 30at the rate of 975 B./D. Gasoline and lighter hydrocarbons werewithdrawn overhead from fractionation zone 30 and passed through a claytower operated at 59 pounds per square inch gauge and then passed to apolymer separator. The clay tower and polymer separator are not shown onthe diagrammatic flow sheet. The conventional clay tower converteddiolens and gum-forming constituents to polymers which were removed fromthe bottom 0E; the polymer separator at a rate of 192 B./D. "The polymerseparator was operated at the top temperature of 353 F. and at apressure of 43 pounds per square inch gauge. The polymer-free gasolineand light hydrocarbons were passed to a gasoline accumulator representedby accumulator 38. VGasoline accumulator 38 was operated at atemperature of 102 F. and at -a pressure of l2 pounds per square inchgauge. Fractionation zone 30 was reiiuxed at a rate of 5380 B./D. withliquid hydrocarbons withdrawn from accumulator 38. A gaseous hydrocarbonfraction was withdrawn overhead from accumulator 38 at a rate of 663B./D. and had the following composition:

Gasoline accumulator overhead C1 76.2 C2 169.7 C2' 22.5 C3 137.2 C3l41.8 C4 71.6 -C4, 13.9 n-.C/L C5' 21.9 C5 39.1 06+ 47.1 Total am) Aliquid hydrocarbon fraction was withdrawn from the bottom of gasolineaccumulator 38 and passed to a gasoline stabilizer represented byfractionation zone d2 at a rate of 3790 B./D. This feed to the gasolinestabilizer had the following composition: Y

Bottoms from gasoline accumulator The gasoline stabilizer represented byfractionation zone 42' was operated with a top temperature of 160 F. andat a top pressure of 9 pounds per square inch absolute. An overheadproduct was Withdrawn from fractionation zone 42 through line 3| at arate 359 B./D. and had the following composition:

Overhead from gasoline stabilizer C2 29 Cz' 3 C3 69 C3' 19 C4 79 -C4' 10Irl-C4' 23 C5 45 C5 82 06+ 0 Total 359 Stabilized gasoline was withdrawnfrom the bottom of fractionation zone 42 at the rate of 3431 B./D.through the line 29. This stabilized gasoline had the followingcomposition:

Stabilizer gasoline "5 B./ D. C4 48 -C4' 8 I1-C4l 1 C5' 'l2 C5 99 C6+3,190

Ttal 3,431

The overhead from gasoline accumulator 38 was combined with the overheadfrom the gasoline stabilizer represented by fractionation zone 42 andpassed to a light hydrocarbon recovery system represented byfractionation zones 54, and 66. The combined streams gave a total of1023 B./D. of light hydrocarbons. This light hydrocarbon feed stream hadthe following composition:

Gasoline accumulator overhead plus gasoline Thus 197 B./D. of C3, C4 andC5 olefins were available to be recovered in the light hydrocarbonrecovery system.

A similar test was then run in which a C3 hydrocarbon stream was addedto gasoline accumulator 38 through line Il at a rate of 500 B./D. Thesame amount of straight run naphtha was reformed, fractionating andheating conditions remained the same and the same amount of fuel,

oil, gas oil, kerosene, polymer and overhead from the de-propanizer werewithdrawn from the system. All towers were refluxed at the same rate.

On overhead gas was withdrawn from the gasoline accumulator 38 at a rate1,162 B./D. This overhead gas stream now had the following composition:

Gasoline accumulator overhead-Cs added C1 76.2 C2 193.6 C2' 25.1 C3466.9 C3' 53.2 C4 107.2 i-C4' 20.3 Il-C4' 32.3 C5' 35.7 C5 62.4 Cs+ 89.5

TOtal 1,162.4

A liquid hydrocarbon stream was withdrawn from the bottom of gasolineaccumulator 38 and fed to a gasoline stabilizer, represented byfractionation zone 42, through line 25 at a rate of 3,790 B./D. Thisgasoline stabilizer feed stream now had the following composition:

Bottoms from gasoline accumulator- C3 added C2 19 C2' 2 C3 139 C3' 15 C4113 -C4' 15' n-C4' 33 C5' 112 C5 172 C6+ 3,170

Total 3,790

The overhead from the gasoline stabilizer now had the followingcomposition and was withdrawn at the rate of 498 B./D.

Overhead from gasoline stabilizer-C3 added C2 19 C2l 2 C3 139 C3' 15 C478 -C4I 10 11-C4l 23 C5' 52 C5 90 C6+ 70 Ttal 498 The stabilizedgasoline withdrawn from the bottom of the gasoline stabilizer now hadthe following composition and was withdrawn at a rate of 3292 B./D.

Stabilized gasoline- C3 added C4 35 -C4' 5 )f1-C4' 10 C5' 60 C5 82 C6+3,100

TOtal 3,292

The overhead from the gasoline stabilizer and the overhead from thegasoline accumulator now totaled 1660 B./'D. and had the followingcomposition.

Gasoline accumulator overhead plus. gasoline stabiliser overhead- Caadded B./D. C1 76 C2 213 Cz' 27 C3 606 C' 68 C4 185 -C4 30 Il-Ci' 55 C5l88 C5 152 l 06+ 160 'IOtal 1,660

Thus the addition of the C3 hydrocarbon stream to gasoline accumulator38 made available 241 B/D. of C3, C4 and C5 oleiins for recovery in thelight hydrocarbon recovery system.

y,The addition of C3 hydrocarbons to the gasoline accumulator increasedthe recovery of C3, C4 and C5 oleiins by 44 B./D., and by 22 per cent.

As will be evident to those skilled in the art, various modifications ofthis invention can be made, or followed, in thelight of the foregoingdisclosure and discussion, without departing from the spirit or scope ofthe disclosure or from the scope of the claims.

I claim:

l. An improved process for recovering C3, C4 and C5 olefins from amixture of hydrocarbons containing the same in admixture with otherhydrocarbons, said mixture being a thermally cracked straight runnaphtha fraction, which comprises passing said mixture `to a firstfractionation zone operating under a pressure of from 300 to 360A poundsper square inch gauge with a top temperature of from 190 F. to 230withdrawing overhead from said first fractionation zone a hydrocarbonfraction comprised predominantly of C2, C3 and C4 hydrocarbons,withdrawe ing a hydrocarbon fraction as bottoms from said rstfractionation zone, passing said withdrawn bottoms to a secondfractionation zone operating under a pressure of from 50 to '70 poundsper square inch gauge and with a top temperature of from 375 F. to 450F., withdrawing overhead from said second fractionation Zone a gaseoushydrocarbon fraction, at least partially condensing same and introducinginto a portion of same a hydrocarbon fraction 'comprised predominantlyof Ca hydrocarbons, passing a resulting hydrocarbon mixture into anaccumulation zone opLrating under a pressure of from 8 to 16 pounds persquare inch gauge and at a temperature oi' from 80 F. to 120 F.,withdrawing a gaseous hydrocarbon fraction overhead from said`accumulation zone, 'withdrawing a liquid hydrocarbon fraction from thebottom portion of said accumulation zone and passing same to a thirdfractionation zone operating under a pressure of from 5 to 13 pounds persquare inch absolute and at a top temperature of from 140 F. to 180 F.,withdrawing a hydrocarbon fraction overhead from said thirdfractionation zone, passing said hydrocarbon fractions, Withdrawnoverhead from said iirst fractionation zone, said accumu lation Zone andsaid third fractionation zone, to a fourth fractionation zone andrecovering therefrom Cs, C4 and C5 olens.

2. An improved process for recovering C3, C4

and C5 olens from a mixture of hydrocarbons constituting effluent from alight oil cracking furnace, which comprises first fractionation zone,withdrawing overhead from said first Afractionation zone a hydrocarbonfractionv comprised predominantly of Cz, C3 and C4 hydrocarbons,withdrawing a hydrocarbon fraction from the bottom portion of said firstfractionation zone comprised predominantly of C4 and heavierhydrocarbons, passing said hydrocarbon fraction withdrawn from thebottom portion of said first fractionation zone to a secondfractionation zone, withdrawing overhead from said second fractionationzone a gaseous hydrocarbon fraction containing gasoline and lighterhydrocarbons not taken overhead in said rst fractionation zone, at leastpartially condensing same and passing a portion ofsaid hydrocarbonfraction withdrawn overhead from said second fractionation zone to anaccumulation zone, introducing into said accumulation zone a lighthydrocarbon fraction containing C3 hydrocarbons and minor amounts ofother light hydrocarbons, withdrawing a gaseous hydrocarbon fractionoverhead from said accumulation zone comprised predominantly of C5 andlighter hydrocarbons, withdrawing a liquid hydrocarbon fraction from thebottom portion of said accumulation zone containing C6 and heavierhydrocarbons and containing appreciable quantities of C5 and. lighterhydrocarbons, passing said liquid hydrocarbon fraction Withdrawn fromthe bottom portion of said accumulation zone to a third fractionationzone, withdrawing overhead from said third fractionation zone ahydrocarbon fraction containing a substantial portion of the C5 andlighter hydrocarbons contained in the above Vmentioned hydrocarbonfraction passed to said third fractionation zone, passing saidhydrocarbon fractions, withdrawn overhead from said rst fractionationzone, said accumulation zone and said third fractionation zone, to afourth fractionation zone and recovering therefrom C3, C4 and C5 olens.

3. An improved process for recovering C3, C4 and C5 olens from a mixtureof hydrocarbons constituting eiiiuent from a naphtha fraction crackingfurnace, which comprises passing said mixture to a first fractionationzone, withdrawing overhead from said first fractionation zone ahydrocarbon fraction comprised predominantly of C2, C3 andC4hydrocarbons, withdrawing a hydrocarbon fraction from the bottom portionof said first fractionation zone comprised predominantly of C4 andheavier hydrocarbons, passing said hydrocarbon fraction withdrawn fromthe bottom portion of said first fractionation zone to a secondfractionation zone, with- .rawing overhead from said secondfractionation zone a gaseous hydrocarbon fraction containing gasolineand lighter hydrocarbons not taken overhead in said first fractionationzone, at least partially condensing same and passing a portion cf saidhydrocarbon fraction withdrawn overhead from said second fractionationzone to an accumulation zone, introducing into said accumulation zone alight hydrocarbon fraction containing C3 hydrocarbons and minor amountsof other light hydrocarbons, withdrawing a gaseous hydrocarbon fractionoverhead from said accumulation zone comprised predominantly of C5 andlighter hydrocarbons, withdrawing a liquid hydrocarbon fraction from thebottom portion of said accumulation zone containing Cs and heavierhydrocarbons and containing apprepassing said mixture to a.

ciable quantities of C5 and lighter hydrocarbons, passing said liquidhydrocarbon fraction withdrawn from the bottom portion of saidaccumulation zone to a third fractionation zone, withdrawing overheadfrom said third fractionation zone a hydrocarbon fraction containing asubstantial portion of the C5 and lighter hydrocarbons contained in theabove mentioned hydrocarbon fraction passed to said third fractionationzone, passing said hydrocarbon fractions, withdrawn overhead from saidfirstl fractionation zone, said accumulation zone and said thirdfractionation zone, to a fourth fractionation Zone and recoveringtherefrom C3, C4 and C5 olefins, alkylating said C3, C4 and C5 olefinsrecovered from the above mentioned three hydrocarbon fractions, passinga resulting alkylation reaction product to a fth fractionation zone,withdrawing overhead from said fifth fractionation zone a hydrocarbonfraction containing C3 hydrocarbons and minor amounts of other lighthydrocarbons, and recycling said hydrocarbon fraction withdrawn overheadfrom said fifth fractionation zone to said accumulation zone.

4. The improved process of claim 2 wherein said light-oil is a naphthafraction.

5. An improved process for recovering C3, C4 and C5 olens from a mixtureof hydrocarbons constituting eiiiuent from a light-oil cracking furnace,which comprises passing said mixture to a rst fractionation zone,withdrawing overhead from said first fractionation zone a hydrocarbonfraction comprised predominantly of C2, C3 and C4 hydrocarbons,withdrawing a hydrocarbon fraction from the bottom portion of said firstfractionation Zone comprised predominantly of C4 and heavierhydrocarbons, passing said hydrocarbon fraction withdrawn from thebottom portion of said first fractionation zone to a secondfractionation zone, withdrawing overhead from said second fractionationZone a gaseous hydrocarbon fraction containing gasoline and lighterhydrocarbons not taken overhead in said rst fractionation zone, removingpolymer forming constituents from same, at least partially condensingsaid resulting gasoline and lighter hydrocarbons and passing resultinghydrocarbons into an accumulation zone, introducing into saidaccumulation zone a light hydrocarbon fraction containing C3hydrocarbons and minor amounts of other light hydrocarbons, withdrawinga gaseous hydrocarbon fraction overhead from said accumulation zonecomprised predominantly of C5 and lighter hydrocarbons, withdrawing aliquid hydrocarbon fraction from the bottom portion of said accumulationZone containing C5 and heavier hydrocarbons and containing appreciablequantities of C5 and lighter hydrocarbons, passing said liquidhydrocarbon fraction withdrawn from the bottom portion of saidaccumulation zone to a third fractionation zone, withdrawing overheadfrom said third fractionation zone a hydrocarbon fraction containing asubstantial portion of the C5 and lighter hydrocarbons contained in theabove-mentioned hydrocarbon fraction passed to said third fractionationzone, passing said hydrocarbon fractions, withdrawn overhead from saidfirst fractionation zone, said accumulation zone and said thirdfractionation Zone, to a fourth fractionation Zone and recoveringtherefrom C3, C4 and C5 olens.

6. The improved process of claim 5 wherein said light-oil is a naphthafraction.

DONALD M. LITTLE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,901,863 Beardsley et al. Mar.2l, 1933 2,387,309 Sweeney Oct. 23, 1945 2,408,753 Burk Oct. 8, 19462,442,440 Scoville June 1, 1948

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